Light-detached resists or reliefs for printing plates



Nov. 27, 1956 M. HEPHER 2,772,160

LIGHT-DETACHED RESISTS 0R BELIEFS FOR PRINTING PLATES Filed March 2L 1952 Fig.1

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ATTORNEYS Unite States Patent LIGHT-DETACHED RESIST?) 0R RELEFS FOR PRHJTTNG PLATES Martin Hepher, Harrow, England, assignor to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey Application Niarch 21, 1952, Serial No. 277,836

19 Claims. (Cl. 96-35) The present invention relates to a photographic method of preparing printing plates for use in lithography, photogravure, and the like.

The object of the invention is to provide a method, and a sensitive sheet for use in the method, for producing either a relief image or a lithographic plate in which the hydrophilic areas are a thin layer of a colloid on a hydrophobic support.

Various methods are known for producing images of this type, some or" which use gelatin containing bichromate or silver halide as a light-sensitive material. Such methods involve the hardening of the layer where exposed to light and then treatment of the layer to remove the unhardened portion leaving an image in hardened gelatin. In general these prior processes involve the formation of a positive image from a negative or vice versa. One object of the present invention is to provide a simple method of producing a positive image from a positive or negative image from a negative.

The present invention is based on the discovery that a gelatin layer containing certain diazonium compounds will become detached from its support upon exposure to light and the exposed portions may be washed off leaving only the unexposed portions. it should be noted that this is contrary to the tanning action sometimes associated with the exposure of diazo compounds to light.

According to the present invention a half tone printing plate is made by coating a support with a gelatin layer containing a diazo compound of the type which causes the gelatin to disengage from the support when exposed to light, exposing the coating to half tone image and then washing off the exposed areas. If the gelatin layer is too hard it will not wash ofi even in the exposed areas. Accordingly, it has been found necessary to have the gelatin at a pH less than 2.5. The simplest way of doing this is to have an acid in the gelatin layer itself to provide the low pH. However, a gelatin layer of higher pH may be used if the aqueous solution used in the washing operation is itself quite acid with a pH below 2.5. Furthermore, it has been found that while gelatin alone gives a useful relief for some processes, it is preferable in all cases and particularly in connection with lithographic processes using an extremely thin layer of gelatin on a hydrophobic support, to include in the gelatine layer an inert comminuted solid such as baryta or other pigment. In this lithographic embodiment the gelatin or gelatin layer plus the comminuted solid should be preferably spread on the hydrophobic support so as to have between .03 and .3 gram per square foot.

When an inert comminuted solid material is incorporated in the gelatin, there should be at least as much solid as gelatin and sometimes up to times as much. For example, the baryta gelatin ratio is preferably between 2 to 1 and 4 to l. The. acid may be of any of the ones usually used in gelatin layers, e. g., hydrochloric, citric, acetic, or oxalic acid.

The gelatin layer also preferably contains a hardening agent such as formaldehyde or chrome alum. The

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amount of hardening agent required, of course, depends on the age of the coating since gelatin itself tends to harden with time. The hardness is preferably equivalent to a gelatin containing between .l% and 2% chrome alum, the larger amounts being necessary with freshly coated gelatin. If no hardener at all is used, a slightly higher pH is permissible, but it has been found that the best results are obtained by adding hardener to the amount here specified and then counteracting this hardening efiect by having a lower pH, i. e., below 2.5.

The term Hardness has an accepted meaning in connection with photographic gelatin; see pages to 126 of the first edition 1942 or pages 73 to 84 of the revised edition 1954 of The Theory of the Photographic Process, by C. E. K. Mees. This term is used herein only in this accepted sense.

Examples of diazonium compounds which are satisfactory for thus purpose and which cause gelatin to become detached from its support upon exposure to light are the diazo derivatives of:

p-Phenylenediamines N-alkyl-p-phenylenediamines N,N-dialkylp-phenylenediamines N-aryl-p-phenylenediamines 4,4-diamino-diphenylmethanes N-alkyl-4,4-diamino-diphenylrnethanes N,N-dialkyl-4,4'-diamino-diphenylmethanes It is noted that p-amino-diphenylamine is one of the N-aryl-p-phenylenediamines. It is also noted that 3- methyl-4-amino-diethylaniline and diethylamino toluidine are examples of N,N-dialkyl-p-phenylenediamines. All of these compounds can be represented by the general formula:

where R and R represent hydrogen, alkyl groups or aryl groups, X represents hydrogen or an alkyl group and n has the value 1 or 2 which means that the bracketed part of the formula is either omitted or is present only once.

It should be noted that the present invention is not primarily concerned with any swelling of the gelatin or any loosening of the gelatin molecules one from the other which would appear as a swelling, but rather is concerned with actual detaching of the gelatin layer from the support. The baryta or other solid is believed to enhance this efiect in the sense that the gelatin acts to form the adhesive between the particles of baryta and hence the detaching action is enhanced when there is only a relatively small amount of gelatin. Also the baryta remaining in the unexposed areas of gelatin tends to enhance its hydrophilic properties in the lithographic embodiments of the invention.

One very special embodiment of the present invention pertains to the making of a printing plate directly from a half tone negative. the invention a processed half tone negative consisting of a silver image in a gelatin layer is bathed in a diazonium compound of the type described above so that the gelatin layer is resensitized. The treated negative is then exposed to light from the front so that the gelatin between the silver dots receives exposure all the way through to the support and thus tends to become detached from the support, whereas those portions of the gelatin behind the silver dots receive essentially no exposure and thus there is no tendency for the silver areas to become detached from the support. The negative is then washed off, preferably with some brushing to remove the gelatin areas between the silver dots and to leave a relief image in gela- According to this embodiment of tin containing silver. The silver may then be removed by a bleach or in some processes it may be left in the gelatin. Also the gelatin may be hardened to withstand repeated printing from the relief by treatment with any suitable hardening agent such as formaldehyde. The solution for washing off the clear gelatin areas in this latter case preferably contains a high concentration of acid and also some hydrogen peroxide which tends to enhance the detaching of the gelatin. It should be particularly noted, however, that this process is just the opposite of the peroxide bleach method which attacks the silver and removes the gelatin in the dot areas and thus gives a positive relief image, whereas the present invention gives a negative relief image, when a half tone negative is treated thereby.

Two embodiments of the invention are illustrated in the accompanying drawing in which:

Fig. 1 is a fiow chart illustrating schematically a process according to the present invention.

Fig. 2 is a similar flow chart illustrating the making of a negative relief image directly from an ordinary half tone negative.

Fig. 1 is intended to illustrate the making of a lithographic printing plate but the illustration is only schematic V:

to remove the silver from the dots 32 by a suitable bleach and would, therefore, be exactly the same for the making of a half tone relief image. The primary difference between the two processes is in the thickness of the sensitive layer, an extremely thin layer being use in the lithographic embodiment. A support which may be metal, film or paper and which may include a subbing 11 is provided with a light-sensitive layer 12. In the lithographic case this support including the subbing 11 has a hydrophobic upper surface and the sensitive layer 12 is hydrophilic so that any areas of the layer 12 which are removed will expose a greasy ink receptive area of the subbing 11 which is considered part of the support for the sensitive layer 12. The sensitive layer is preferably made up of gelatin, baryto, a small amount of hardener which is to be overcome by having either the layer or the washing solution highly acid and a diazo compound of the type discussed in detail above.

The second step illustrated in Fig. 1 shows the sensitive layer 12 behind a ruled half tone screen through which an image of the object 13 is focused by a lens 14 forming a half tone image on the sensitive layer 12. This half tone image tends to cause the gelatin layer 12 to become detached from the subbing 11. The detached areas are then washed off by immersing the film in a container 21 containing an acid solution and brushing with a brush 22 or swabbing with cotton. Alternatively, if the layer 12 contains an acid so that it has a low pH, the solution 20 may simply be water or the washing operation can be accomplished by placing the film under a tap so that the Water 23 flows thereover as illustrated in the alternative embodiment of the third step of the processing. On the other hand, the pH of the layer 12 may be as high as 5 or 6 providing the solution 20 has a pH below 2.5. In practice it is common to have the layer 12 with a pH below 1. After Washing out the areas of the layer 12 which have been exposed to a half tone image, the resultant plate consisting of support 14) with a hydrophobic layer 11, has hydrophilic areas 25 thereon and hydrophobic exposed areas 26 which receive the printing ink. Quite satisfactory results are obtained by using a sheet of paper for the support 10 with cellulose nitrate as the hydrophobic subbing 11 and a gelatin baryta layer thereon as discussed in detail above. Such paper plates can be mounted in standard lithographic presses and even with the simplest form of the invention, using extremely thin layers of gelatin, several 100 prints may be run from such a plate with definitely good quality.

If a relief half tone image is desired, it is, of course, necessary to use a thicker gelatin layer 12 so that the dots 25 are correspondingly thick to be used either in imbibition printing or in letter press although the quality of the resulting prints in this case are in general not as good as those the lithographic embodiment gives.

Fig. 2 illustrates an ordinary half tone negative on film base 30 having a processed gelatin layer 31 containing silver dots 32 and clear gelatin areas 33 between the dots. According to the invention this half tone negative is bathed in a solution 35 containing a diazonium compound of the type discussed above and preferably containing acid to bring the pH of the gelatin down. In general it appears best to bring the pH down only to about 3 and to depend on the acid in the ultimate wash solution to ensure that the pH is proper at the washing stage. After the treatment in diazo solution, the negative is exposed to intense light, represented by arrows 36, from the front. The interface bctween the support 30 and the gelatin layer 31 includes areas 37 which receive this intense exposure 36 and areas 38 which are in the shadow of the silver dots and hence are not exposed to any effective degree. The negative is then immersed in an acid solution 40 in a dish 41 and swabbed or brushed with a brush 42 to remove the exposed areas and to leave the silver gelatin dots in areas 32 as shown in the final step of the flow diagram.

For imbibition printing from this plate it is preferable prior to immersing the plate in the dye bath.

By way of specific examples of the present invention but not limited thereto, the following materials and process are listed.

; the chrome alum is about 3 or 4 percent of the gelatin content and that the coating contains about .8 gram of gelatin per square foot. When the plate is dry, it is bathed for one minute in the following diazo solution:

3-methyl-4-amino-diethylaniline grams 10 Hydrochloric acid (concentrated) cc 10 Sodium nitrite grams 3.3 Water (kept below 40 F.) cc 200 The sodium nitrite, among other things serves as a diazotizing agent. The surplus liquid is then wiped off .and the material dried. An exposure can then be made in contact with a line negative to two 400-watt high pressure mercury vapor lamps for 6 minutes, and the print then washed in running water at a temperature of F., whereby the exposed portions are removed leaving the unexposed portions as a relief on the glass plate.

Example 2.The following examples and several variations thereof herein mentioned show the application of the invention to the production of a presensitized, easily processed, lithographic material suitable for ultimate use on ofiice lithographic printing machines. A paper base is first dope coated with cellulose nitrate lacquer, in one embodiment it is coated on both sides or alternatively it may be coated only on the side to receive the sensitive layer. The cellulose nitrate subbing is then coated with a thin layer of gelatin and baryta containing 4 parts baryta to 1 part gelatin and 0.01 part of chrome alum which corresponds to 1 percent of the gelatin content and is spread on the base about 1.2 grams per square foot. The barytagelatin layer is then sensitized by swabbing with the diazo solution mentioned in Example 1. The excess is wiped off and the layer dried. Exposure is then effected under a negative for 6 minutes to two 400-watt high pressure mercury vapor lamps at 18 inches. The sheet is washed under a cold running tap and wiped with cotton cloth whereby the exposed baryta regions are removed.

After removal of excess moisture this lithographic plate is kept in a moist condition to maintain the best hydrophilic condition of the gelatin-baryta areas while the cel- I lulose nitrate image areas are good ink receptors. The

gelatin-baryta areas can be more strongly hardened for long printing runs by treatment with an alkaline solution,

or by a treatment of solutions of zirconium salts. Furthermore, retouching or modifications of the image can be applied to this plate by typing thereon with a greasy ribbon or by applying lithographic ink. This is preferably effected after the surface has been thoroughly hardened as just mentioned. As a variation of this second example, coatings of .04, .10, .21, .275, and .47 gram per square foot have been coated. For lithographic purposes the .04 and the .10 gram per square foot prove to give somewhat better results than the thicker layers and to be adequate for printing runs of a few hundred. Incidentally, the sensitized plate appears yellowish in color but the color disappears in the exposed areas. The hardness of the gelatin depends on how long it is allowed to age after being coated on the paper and on the quality of the gelatin itself as well as the amount of hardening agent added. The high quality gelatins tend to adhere better in the unexposed areas and low quality gelatins to detach better in the exposed areas in somewhat the same way that the presence of baryta tends to improve the detaching qualities.

The base of paper appears to be quite satisfactory, but, of course, metal foil or film or laminated plastics may be used. With paper, cellulose nitrate subbing is quite satisfactory but any hydrophobic material which allows a layer of gelatin to be firmly attached to it is equally useful. The effect of any hardening depends on the particular agent which is used. For example, a very small amount of chrome alum has quite a large effect especially if the gelatin is allowed to stand a few days. Accordingly, the hardness is best defined using some standard gelatin or at least gelatin containing standard amounts of chrome alum as the reference. Using a high quality gelatin in order to get the best adhesion and applying plenty of water for moistening during the printing operation, it has been found that these paper plates can be used in offset printing giving quite satisfactory prints for runs of several 100. The hardener is defined as a percentage of the gelatin content instead of the total gelatin-baryta content since, of course, it is primarily pertinent to the gelatin.

Examples of patent diazo compounds which have proven satisfactory in this particular lithographic example are the diazo derivatives of the following:

Diethyl-p-phenylenediamine (hydrochloride) Diethylarnino-toluidine (hydrochloride) 3-methyl-4-amino-diethylaniline 4-4'-diamino-diphenylmethane p-Amino-diphenylamine Parts Gelatin 5 Baryta Chrome alum 0.2 Water 100 When dry, these plates are left to mature for one day and then bathed in a 3 percent solution of the tetrazo compound produced from 4-4-diamino-diphenylmethane. The pH of the solution is brought to approximately 0.6 by the addition of an acid, either a mineral or organic acid, e. g., hydrochloric or oxalic acid. In the present application the term diazo compound is used to include tenazo compounds. After bathing in the sensitizing solution, the print is drained and dried in a warm current of air. After exposure through a half tone negative to a mercury vapor lamp or to an are (e. g., 5 minutes at 18 inches from two 400-watt high pressure mercury Parts Hydrogen peroxide 5 Hydrochloric acid (concentrated) 1 Water 30 Example 4.The following emulsion is coated onto subbed cellulose acetate sheets:

Parts Gelatin 2.5 Formaldehyde 0.1 Water This is coated at approximately 5 cc. per square foot and when dry is allowed to mature for one day. It is then bathed in a 3 percent solution of the diazo compound of p-amino-diphenylamine which has been adjusted to a pH of 1.5. After exposure as described in Example 3, the exposed areas may be washed away with cold water.

Example 5 .-A line or half tone negative produced on a normal high contrast processed plate is bathed in a 3 percent solution of the diazo salt of p-phenylenediamine which is adjusted to a pH of 3.0. The excess solution is wiped off and when dried the negative is exposed from the front, i. e., with the emulsion side facing the source of illumination, either to a carbon arc or a mercury vapor lamp (e. g., 6 minutes at 15 inches from two 400-watt mercury vapor lamps). It should be noted that the pH has not been brought all the way down to 2.5 and, therefore, it is necessary to wash the exposed plate in an acid solution. After exposure the clear gelatin parts of the negative are therefore washed away in a solution of hydrogen peroxide, 5 parts; hydrochloric acid (concentrated), 1 part; water, 30 parts. It will be noted that this gives a negative relief image from a half tone negative whereas systems employing a peroxide bleach of the silver image produce a positive image from a negative.

Thus several examples of my invention have been set forth in detail, but it is to be understood that the invention is not limited to these examples but is to the scope of the appended claims.

I claim:

1. A photosensitive sheet comprising a support and a layer on the support made up of gelatin containing suflicient acid to a have a pH below 2.5, between 0.1 percent and 2 percent chrome alum and a diazo derivative of the compounds selected from the group consisting of p-Phenylenediamines, and 4,4'-diamino diphenylmethanes 2. A photosensitive sheet comprising a support and a layer on the support made up of gelatin and an inert comminuted solid material in a ratio between 1:1 and 1:10, said layer containing sufiicient acid to have a pH below 2.5, between 0.1 percent and 2 percent chrome alum and a diazo derivative of the compounds selected from the group consisting of p-Phenylenediarnines, and 4,4'-diamino diphenylmethanes 3. A photosensitive sheet comprising a support and a layer on the support made up of gelatin and baryta in a ratio about 1:4, said layer containing suflicient acid to have a pH below 2.5, chrome alum in an amount between 0.1 percent and 2 percent of the gelatin and a diazo derivative of the compounds selected from the group consisting of p-Phenylenediamines, and

4,4'-diamino diphenylmethanes 4. A photosensitive sheet comprising paper coated with cellulose nitrate and a thin layer, between .03 and .3 gram per square foot, on the support made up of gelatin and baryta in a ratio about 1:4, said layer containing sufficient acid to have a pH below 2.5, chrome alum in an amount between 0.1 percent and 2 percent of the gelatin and a diazo derivative of the compounds selected from the group consisting of p-Phenylenediamines, and

' 4,4'-diamino diphenylmethanes 5. The method of making a halftone printing plate which comprises coating a support with a gelatin layer containing a. diazo derivative of the compound selected from the group consisting of p-phenylenediamines and 4,4diamino diphenylmethanes, exposing the coating to a halftone image and washing completely off the support the disengaged exposed areas of the coating at a pH more acid than 2.5.

6. A photosensitive sheet comprising a support and a layer on the support made up of gelatin containing sufficient acid to have a pH below 2.5, between 0.1 percent and 2 percent chrome alum and a diazo derivative of N-alkyl-p-phenylenediamine.

7. A photosensitive sheet comprising a support and a layer on the support made up of gelatin containing sulficient acid to have a pH below 2.5, between 0.1 percent and 2 percent chrome alum and a diazo derivative of N,N-dialkyl-p-phenylenediamine.

8. A photosensitive sheet comprising a support and a layer on the support made up of gelatin containing suificient acid to have a pH below 2.5, between 0.1 percent and 2 percent chrome alum and a diazo derivative of N-aryl-p-phenylenediamine.

9. A photosensitive sheet comprising a support and a layer on the support made up of gelatin containing sufficient acid to have a pH below 2.5, between 0.1 percent and 2 percent chrome alum and a diazo derivative of N-alkyl-4,4'-diamino diphenylmethane.

10. A photosensitive sheet comprising a support and a layer on the support made up of gelatin containing sufiicient acid to have a pH below 2.5, between 0.1 percent and 2 percent chrome'alum and a diazo derivative of N,N-dialkyl-4,4'-diamino diphenylmethane.

11. A photosensitive sheet comprising a support and a layer on the support made up of gelatin and an inert comminuted solid material in a ratio between 1:1 and 1:10, said layer containing suflicient acid to have a pH below. 2.5, between 0.1 percent and 2 percent chrome alum and a diazo derivative of N-alkyl-p-phenylenediamine.

12. A photosensitive sheet comprising a support and a layer on the support made up of gelatin and an inert comminuted solid material in a ratio between 1:1 and 1:10, said layer containing suficient acid to havea pH below 2.5, between 0.1 percent and 2 percent chrome alum and a diazo derivative of N,N-dialkyl-p-phenylenediamine.

13. A photosensitive sheet comprising a support and a layer on the support made up of gelatin and an inert comminuted solid material in a ratio between 1:1 and 1:10, said layer containing sufiicient acid to have a pH below 2.5, between 0.1 percent and 2 percent. chrome alum and a diazoderivative of N-aryl-p-phenylenediamine.

14. A photosensitive sheet comprising a support and a layer on the support made up of gelatin and an inert comminuted solid material in a ratio between 1:1 and 1:10, said layer containing suflicient acid to have a pH below 2.5, between 0.1 percent and 2 percent chrome alum and a diazo derivative of N-alky1-4,4'-diamino diphenylmethane.

15. A photosensitive'sheet comprising a support and a layer on the support made up of gelatin and an inert comminuted solid material in a ratio between 121 and 1:10, said layer containing sufiicient acid to have a pH below 2.5, between 0.1 percent and 2 percent chrome alum and a diazo derivative of N,N dialkyl-4,4-diamino diphenylmethane. V

16. The method of making a halftone printing plate which comprises coating a support with a gelatin layer containing an inert comminuted solid and a diazo ,derivative of the compound selected from the group consisting of p-phenylenediamines and 4,4-diamino diphenylmethanes, exposing the coating to a halftone image and washing completely ofi the support the disengaged exposed areas of the coating at a pH more acid than 2.5.

17. The method according to claim 16 in which said washing is with an aqueous solution having a pH more acid than 2.5.

18. The method of making a lithographic halftone printing plate which comprises coating a hydrophobic support with a thin gelatin layer, between .03 and .3 gram per square foot, containing an inert comminuted solid and a diazo derivative of the compound selected from the group consisting of p-phenylenediamines and 4,4-diamino diphenylmethanes, exposing the coating to a halftone image and washing completely off the supportthe disengaged exposed areas of the coating at a pH more acid than 2.5. a

19. The method of making a halftone printing plate from an exposed and processed halftone silver imagein a gelatin layer which comprises bathing the gelatin layer in a solution of a diazo derivative of the compound selected from the group consisting of p-phenylenediamines and 4,4-diamino diphenylmethanes, exposing the treated gelatin layer from the front so that areas adjacent to the support and behind the silver image are essentially unexposed and washing completely olf the support the dis engaged exposed areas between the silver image areas.

References Cited in the file of this patent UNITED STATES PATENTS w um 

1. A PHOTOSENSITIVE SHEET COMPRISING A SUPPORT AND A LAYER OF THE SUPPORT MADE UP OF GELATIN CONTAINING SUFFICIENT ACID TO A HAVE A PH BELOW 2.5, BETWEEN 0.1 PERCENT AND 2 PERCENT CHROME ALUM AND A DIAZO DERIVATIVE OF THE COMPOUND SELECTED FROM THE GROUP CONSISTING OF P-PHENYLENEDIAMINES, AND 4,4''-DIAMINO DIPHENYLMETHANES 